A related control device for a variable capacity pump for construction machinery such as a hydraulic shovel, etc., was described by this applicant in Japanese Patent Application No. Hei. 7-46508 (1995). This construction machinery control device is applied to a variable capacity pump including an engine; a variable capacity pump, driven by the engine; a pump output controller, for exerting control in such a manner that the product of the load pressure and the discharge amount at the pump becomes approximately fixed; an operating device, operated by an actuator receiving pressurized oil from the pump; and a switch, for selecting engine output torque and variable capacity pump absorption torque, depending on the operating site or the operation contents. The control device includes an active mode selector, for demanding operations such as heavy excavating, etc.; an engine fuel injection position setting means, for supplying fuel so that the engine outputs the rated output torque in response to the active mode selection; an active mode switcher means, for switching over pressure settings of relief valves and safety valves, etc., for adjusting oil pressure going to the actuator during an active mode; and a controller, for receiving a signal from the active mode selector and for outputting an instruction to the engine fuel injection position setting means and the active mode switcher.
The control device for a variable capacity pump determines the hydraulic pump discharge flow rate Q (Q=q(cc/rev).N) corresponding to the engine speed N, exerts control in such a manner that the product of the discharge flow rate Q and the discharge pressure P of the hydraulic pump becomes constant (P.Q=constant), and controls the hydraulic pump absorption horsepower to be approximately constant.
In recent years, changes have been made to these P-Q charts and to matching points of the engine output torque and the pump absorption torque in order to increase the operating power and the speed in line with the load conditions of the operations.
However, in order to exert control so as to select either one of the point of view of the amount of work or the point of view of the fuel conservation in response to the operating conditions, the control device for a variable capacity pump of Japanese Patent Application No. Hei. 7-46508 carries out a variable control of the engine output, a variable control of the pump absorption torque, and a variable control of the pressure increases in an oil pressure circuit. The control device is therefore complicated, which means that the durability of each control system has to be increased to cope with the vibrations of the construction machinery, which causes costs to increase.
However, various kinds of construction equipment, such as hydraulic shovels, etc., from large types to small types, are used in a wide range of applications from work quarrying mines to municipal engineering work, with both the usage at work sites and the work loads being different.
The type of equipment for carrying out heavy load operations requires a variable control of the engine output, a variable control of the pump absorption torque, and a variable control of the pressure increases in an oil pressure circuit. However, with general engineering work where there is little operation with a heavy load, the aforementioned variable control of the engine output and the variable control of the pressure rises in an oil pressure circuit are no longer required; and a simple control, where a control is exerted to make pump absorption torque a prescribed value in accordance with increases or decreases in engine speed, can instead be carried out.